Industrial chemical vessel



Patented Jan. 19, 1932 PATENT: OFFICE RALPH. H. MCKEE, F JERSEY CITY, NEWJ ERSEY INDUSTRIAL CHEMICAL VESSEL No Drawing.

Application filed March 5, 1929. Serial No. 844,604.

This invention relates to industrial chemis well known, is expensive and also is not ical vessels subjected to destructive action of chemical and physical forces or. agents, and more particularly to pulp digesters used in the paper making industry.

It is well known that sulfite paper pulp is strong enough mechanically for digester purposes. The bronze digesters were not satisfactory from the safety point and, as a matter of fact, there were a number of deaths due to explosions of such digesters. Consemade by heating wood chips with a solutionrquently, the art desired to replace these unof sulfur dioxide in Water. This solution carries in addition some calcium bisulfite, (Ca(HS03)2), and is commonly called sulfitef cooking liquor. The heating of the wood chips in the aforesaid solution is continiued for about six to twelve hours and the temperature is kept at about 145 to 155 C. A part of the sulfur dioxide is taken off with the steam vapors and recovered for re-use. The lignin goes into solution and is ordina'rily removed by draining or filtering, as is well understood in the art. Under ordinary coinditions the lignin solution is usually discajrded into the sewer. The residual cellulose or pulp remaining after the withdrawal of thelignin solution is washed, screened and bleached and is then ready to make paper.

In carrying out the aforesaid process of making sulfite paper pulp large chemical vessels, commonly termed digesters, were utilized. One of the first types of digesters used was a steel digester lined with'sheet lead.

This type of "construction proved to be unsatisfactory for a variety of reasons. For instance, the lead sagged in the digester due to its enormous weight. A further serious defect of the lead-steel digester was that when the digester was heated it expanded and upon cooling it did not contract to the same extent. Although lead was chemically resistant to the sulfurous acid solution, that is the sulfite cooking liquor, the lead was not a desirable material from the mechanical standpoint as is obvious from the aforemen-' tioned defects. Any leakage that occurred brought the sulfite cooking liquor into direct contact with the outer steel wall which was consequently subjected to rapid corrosion.

In order to avoid the disadvantages and defects of the lead-lined steel digesters, another type of digester was proposed. The proposed type of sulfite-digester was one of bronze plates riveted together. Bronze, as

safe digesters with a safe apparatus which would withstand the destructive and deteriorating chemical and physical forces involved in the sulfite. pulp process.

The art replaced the bronze digesters with the presenteday digester, which is made of ordinary carbon steel riveted together. The steel digester has a lining of cement, and this cement lining is further lined with acid-resistant brick. The customary material whichcements the bricks together and to the cement lining is generally a litharge-glycerin-silica cement. As is well known, such brick linings are difficult to keep in repair and have to be constantly repaired. Usually repairs have 1 to be made on them every week or two in orderto keep them in satisfactory condition. These dlgesters are costly to install and expensive to maintain and have their capacity decreased by about one-quarter due to the relatively thick cement and brick lining.

Many attempts have been made to remedy the defects of the brick-lined steel digesters and to replace them with more satisfactory types of apparatus. Up to the present, hoW- ever, the various attempts have been in vain because of the severe burdens and r uirements imposed upon apparatus intended for use as sulfite digesters. As is well known, a digester is about 40 feet in height and about 12 feet in diameter. The bottom of the digester has asloping contour to permit the blowing or removal of the pulp through an outlet at the lower end thereof while the top has a charging inlet and also other appro' priate auxiliarieswell known inthe art. This huge digester, in addition to being subjected to the chemical and destructive corrosive action of the sulfite cooking liquor, the lignin, reaction products, and the like, is subjected to enormous physical stresses such as steam pressure, alternate cooling and heating, relatively high temperatures, structural stresses,

which might serve for sulfite digesters. It is known that cast iron is somewhat more resistant to acids than is ordinary steel, such as is used at present for the outer shell of the digesters. However, cast iron cannot really be considered as a material for making digesters because it is readily attacked by sulfurous acid and also is not strong enough mechanically. Moreover, it will not withstand the expansion and contraction due to the alternate heating and cooling of the digester without cracking. Similar to cast iron are high silicon irons, such as duriron, which are quite resistant to nitric and sulfuric acids, but which are not resistant to sulfurous acid.

I Furthermore,.they expand greatly with heat and, in addition, are brittle. Like cast iron, these silicon irons are unable to withstand the mechanical strains incident to the use of such material in making sulfite digesters.

Cast iron and silicon irons, consequently, fail to meet the requirements of sulfite pulp digesters.

Subsequently, copper digesters were proposed but copper was neither strong enough mechanically nor resistant to the sulfite cooking liquor. For example, by experiments which I carried out, it was found that copper was dissolved by hot sulfite liquor at the rate of about two pounds per square foot per year.

Attempts were made to utilize nickelsteels and it was found that nickel steels were attacked somewhat more slowly than ordinary carbon steels. The rate of the corrosive attack was found to be approximately the same as for copper, that is, about two pounds per square foot peryear. This rate of corrosion was far too great to permit the use of the material for digesters even though the nickel steels were strong enough mechanically.

In recent years plating of ordinary steel with resistant material such as chromium has produced articles which apparently were quite resistant to corrosion under certain conditions. It was thought that a chromium lated digester might function satisfactorily. pon further consideration, however, it was .realized that chromium plating was expensive and was difiicult to produce. The plating, as is well known, is only of a superficial thickness, and, hence, readily removed by.

mechanical abrasion. Another objection is that the plating does not cover the edges ,of the steel plates, if the plates were plated before being installed, in the digester. Furthermore, such plates give very serious troucrazes and flakes off. Chromium plating on steel which is alternately heated and cooled is not a protection against corrosion from sulfite cooking liquor, as the solution works under the plate and soon loosens the plating and removes all protection.

Various alloy steels were suggested as being appropriate for the construction of sulfite digesters, but none proved to be commercially successful and satisfactory. For example, in my experiments with alloy steels of the stainless steel type, I ascertained that they were readily pitted by sulfurous acid. Steels of this type carrying chromium Would not withstand the destructive corrosive action of sulfite cooking liquor. The same thing was true of the aforesaid types of steel which carry 20% of chromium. The cooking liquor attacked the surface of these metals as a whole, and,'even more important,

produced pits in the surface. Holes as large as a quarter of an inch deep were produced in plates in a ten week period. The afores a1d types of chromium steels were a dlsappolntment and a failure in so far as sulfite digesters were concerned.

After extended and prolonged investigations, I have discovered, however, that, contrary to expectations, certain particular types of alloy steels having special constitutions arecapable of being fabricated as durable, efiicient and desirable sulfite pulp digesters in a practical, satisfactory and commercial manner. I have found that certain alloy steels which contain about 15% or more but less'than about of chromium and which contain about one half as much nickel (i. e., from about 7.5% to about 10%) possess suitable resistivity and durability with respect to resisting the corrosive and destructive action of sulfite cooking liquor and of oxidizing and electrolytic forces, and, at the same time, possess requisite strength, toughness and hardness with respect to withstanding the physical forces of pressure, particularly steam pressure, of relatively high temperatures, of alternate heating and cooling, of structural stresses due to the huge size of sulfite digesters, of abrasion, and of other agents to which sulfite digesters are subjected. These particular alloy steels can be made into commercial plates and also rivets from which a sulfite digester can be fabricated without deleteriously affecting their properties of'resisting diverse chemical actions and withstanding physical forces noted hereinabove.

In practice, I prefer, mainly for commercial reasons, to utilize an alloy steel contain- .lng about 18% chromium, about 8% nickel.

sels.

composition, or the steel can be welded with-' I out difliculty to form industrial chemical ves- The aforementioned alloy steel, contrary to experience with other alloy steels of a similar nature, is sufiiciently durable to withstand disintegration, while at the same time, it

possesses to a high degree-,the desired propertyof withstanding physical and thermal forces present in operating a sulfite pulp digester. In these particular alloy steels, I have, therefore, discovered materials which by reason of their peculiar constitution are especially suitable for fabrication as sulfite digesters and for reliably andsuccessfully enduring the disintegrating efi'ects prevailing under operating conditions. For instance, I have carried out-experiments with such alloy steels in hot sulfite cooking liquor for long periods of time, for example, exposure for ten weeks, without'prceptible effect on the surface of the steels nor loss of weight that was appreciable. Suchsteels are stronger than ordinary carbon steel such as is customarily used and, in consequence, the thickness of the digester walls made of alloy steels would need to be approximately 60% as much as used in the making of the present digester shells. Such alloy steels, as I have specified, have a thermal conductivity of only about one third that of ordinary steel. Consequently, the heat insulation need not be as thick or expensive as it would be for a digester made of ordinary steel. It is these new and unexpected properties of the aforementioned alloy steels which, thus, make them peculiarly suitable for the purposes of fabricating sulbe decreased from the lower limits of the figures given by as much as one third of the percentages indicated without the product losing its usefulness, for the purpose designed almost completely nor, on the other hand, can the percentages of the other elements, except iron, chromium and silicon, be increased by as much as one third of their indicated percentages without also having the effect of rendering the material unsuitable for the purpose indicated. Even with'chromium at 18% I and the other constituents right, if nickel is omitted or decreased considerably in amount,

the steel is useless in that it will not stand the chemical action of the sufite cooking liquor. It is to be noted, therefore, that the alloy steels have to be constituted of special elements and that the constituents have'to be present in special percentages within a given and therefore are wasted.

critical range, otherwise the alloy steels are practically commercially useless or without very much industrial merit in so far as sulfite pulp digesters are concerned.

= A digester made of the alloy steels noted hereinabove need contain no inner lining of cement and brick, but advantageously could be lagged with some heat-insulating material on the outer surfaces thereof to decrease loss of heat by radiation. If made of the same size as the ordinary digester, it'would have a capacity about one quarter more, or if a digester had the same capacity, it could be made of smaller diameter with consequent decrease in the number of square feet of steel plates used, that is, with decrease in the amount of material used due tothe decreased area as well as decreased thickness. In other words,

i the area of the plates used would be decreased and the total weight of the digester would be materially decreased. Moreover, such a digesterwould be smooth on the inside and in bottom of the digester more nearly flat instead of conical with a consequent saving of material and labor, or if made in the same shape as at present, it would permit the re- .moval of sulfur dioxide and steam vapors to a lower pressure than that now used with resulting saving of sulfur consumption. Any sulfur dioxide removed through the relief pipes of the digester in relieving the pressure is recovered for re-use in the process, but steam and sulfur dioxide blown out with the chips cannot readily be recovered for re-use The reduction of pressure to about thirty pounds before blowing is the customary procedure at the present time.

An important feature of my sulfite pulp digester made of the alloy steels which I have discovered is that the pulp made in such a digester will have an appreciable lower ash content than at present, and, in consequence, will be more easily bleached than the pulp which is made in the priorand well known cement and brick-lined steel digesters. In the manufacture of pulp in these cement and brick-lined steel digesters, the pulp is in contact with the lining and abrades particles of material from such lining, so that the pulp is contaminated and has its ash content increased. With my improved sulfite digester thesurface of the metal keeps a good polish even when subjected to corrosive fumes and oxidizing influences and to corrosive sulfite cooking liquors and to thermal effects.

Although I have mentioned a specific composition as being preferred for the fabrication of sulfite pulp digesters merely by way of illustration, it is to be understood that I am not to be confined to the exact constituents mentioned in said "preferred example as they are to be regarded only as typical and illustrative and that my invention extends to the limits of the critical ranges which I have described and which I have set forth in the claims; Various modifications may be made Without departing from the nature of my invention as defined in the appended claims.

I claim:

1. A sulfite pulp digester composed of a steel alloy having a low carbon, phosphorus, and sulfur content and containing a'metal having the properties of chromium in the proportion of at least approximately 15% or more but less than about 20% and a metal having the properties of nickel in the proportion of about at least 7.5% or more but less than about 10%, said digester being capable of resisting alternations of temperature, corrosive and destructive action of the sulfite liquor and fumes and physical stresses due to the thermal and pressureconditions involved in sulfite pulp digestion.

2. A sulfite pulp digester composed of a steel alloy containing about 18% chromium and about 8% nickel, said digester being capable of resisting alternations of temperature, corrosive and destructive action of the sulfite liquor and fumes and physical stresses due to the thermal and pressure conditions involved in sulfite pulp digestion.

3. A sulfite pulp digester composed of a steel alloy having a carbon content un er 0.20%, phosphorus under 0.05%, and sulfur under 0.05% and containing approximately 18% chromium, approximately 8% nickel,

about 0.50% manganese and about 0.50% silicon, said digester being capable of resisting alternations of temperature, corrosive and destructive action of the sulfite liquor and fumes and physical stresses due to the I thermal and pressure conditions involved in sulfite pulp digestion.

In testimony whereof I have hereunto set my hand.

RALPH H. MoKEE. 

